Demands for secondary batteries as an energy source have rapidly increased as technology development and demands for mobile devices have increased. In addition, among such secondary batteries, studies on lithium secondary batteries having high energy density and a discharge voltage have been extensively carried out, and lithium secondary batteries are commercialized and widely used.
Secondary batteries are divided into, depending on the shape of a battery case, a cylindrical battery and a rectangular battery in which an electrode assembly is built in a cylindrical or rectangular metal can, and a pouch-type battery in which an electrode assembly is built in a pouch-type case of an aluminum laminate sheet.
Generally, a cylindrical secondary battery includes a can, an electrode assembly housed inside the can; a cap assembly combined to an upper opening of the can; and a washer installed on an upper part of the cap assembly.
In addition, in the cylindrical secondary battery, an outer surface of the can is coated (tubed) with an insulating outer cover to obtain an effect of protecting a battery cell from external shock and complementing mechanical strength of members installed on the upper part of the battery cell while performing a function of insulation, a function of can fixation, a function of lot number printing and the like. Moreover, stability of the battery cell may be enhanced by preventing an internal short circuit of a battery capable of being caused by corrosion of the battery cell and corrosion of electronic devices such as a protection circuit module (PCM).
Generally, tapes or tubes formed with a heat shrinkable resin such as a PET material haven been used for coating an outer surface of a can of a cylindrical secondary battery. However, tubing is readily broken from consistent shock or instantaneous strong shock during a Drop test, and rust also occurs when exposed to a harsh atmosphere of high temperature and humidity, which leads to a problem of declining a lifespan and stability of a secondary battery.
In order to solve such a problem, improvements such as development of tubing materials, design changes in tubing or methods of insulating a secondary battery surface other than tubing have been required.